1. Field
Embodiments of the present disclosure relate to flip chip bonding of chips constituting an ultrasound probe.
2. Description of the Related Art
Ultrasound diagnostic devices operate to obtain a cross-sectional image of a soft tissue or bloodstream in a non-invasive manner by irradiating an ultrasound signal through the surface of a subject to a target site inside the subject and receiving an ultrasound echo signal reflected from the target site.
The ultrasound diagnostic devices are smaller in size and less expensive than other image diagnostic devices (e.g., X-ray diagnostic device, computerized tomography (CT) scanner, magnetic resonance imaging (MRI), nuclear medicine diagnostic device, etc.). In addition, the ultrasound diagnostic devices may enable real-time display of a diagnostic image and are safe because there is no risk of exposure to X-rays. Thus, these ultrasound diagnostic devices are widely used in diagnosis in obstetrics and gynecology, diagnosis for the heart and abdomen, and urology diagnosis.
An ultrasonic diagnostic device includes an ultrasound probe to transmit an ultrasound signal to a subject and receive an ultrasound echo signal reflected from the subject to obtain an ultrasound image of the subject.
In general, the ultrasound probe includes an ultrasound transducer in which a plurality of piezoelectric crystal elements (i.e., piezoelectric vibrators) are arranged on a plane in a matrix or array form, and the piezoelectric crystal elements perform an interactive conversion between electric energy and mechanical vibration energy to transmit and receive an ultrasound signal.
Recently, a new concept of a non-contact ultrasound transducer, i.e., a capacitive micromachined ultrasonic transducer (cMUT) has been developed, which enables high-efficient transmission and receipt of an ultrasound.
The cMUT is a relatively new type of an ultrasound transducer that transmits and receives an ultrasound using vibration of hundreds of or thousands of micromachined thin films, and is manufactured based on micro electro mechanical systems (MEMS) technology. When thin films having a thickness of thousands of Å are formed on a semiconductor substrate used in a general semiconductor manufacturing process with being separated from each other with an air gap having a thickness of thousands of Å, the semiconductor substrate and the thin films form a capacitor with the air gap formed therebetween.
When alternating current flows to the manufactured capacitor, the thin films are vibrated and, consequently, an ultrasound is generated. By contrast, when the thin films are vibrated by an external ultrasound, a capacitance of the capacitor is changed and the change in capacitance of the capacitor is detected, thereby receiving an ultrasound.
A single cMUT has a diameter of only tens of micrometers and thus, even though ten thousands of cMUTs are arranged, the size thereof is only several millimeters. In addition, ten thousands of sensors may be accurately arranged at a desired position simultaneously using a one-time manufacturing process and thus the accuracy is incomparably superior to an array sensor using a piezoelectric sensor.
To transmit an electric signal to these cMUTs, the cMUTs need to be connected to an integrated circuit such as an application specific integrated circuit (ASIC) using a chip bonding method such as flip chip bonding.
Flip chip bonding is a technology by which solder balls (solder bumps) are formed on a semiconductor chip on which an integrated circuit is formed, to be electrically connected to the integrated circuit, and the semiconductor chip is directly mounted on a substrate using the solder balls. The flip chip bonding process may enable mounting of a semiconductor chip using solder balls and electrical connection through the solder balls and provide a short electrical path, and thus may be widely used in manufacturing electronic products that require miniaturization, light weight, and high-density mounting.
FIGS. 1A and 1B are diagrams for explaining a general flop chip bonding process.
To perform flop chip bonding of two chips, first, as illustrated in FIG. 1A, a surface of a first chip 10 on which bonding pads 12 are not formed is suctioned using a vacuum suction device 30 and a surface of a second chip 20 on which solder balls 22 are not formed is suctioned using a vacuum suction device 40 such that a surface of the first chip 10 on which the bonding pads 12 are formed faces a surface of the second chip 20 on which the solder balls 22 are formed. Subsequently, the solder balls 22 formed on the second chip 20 are dipped in a flux, and, as illustrated in FIG. 1B, the second chip 20 is pre-adhered to the first chip 10 by aligning the second chip 20 with the first chip 10 so that the solder balls 22 of the second chip 20 contact the bonding pads 12 of the first chip 10. Thereafter, the vacuum suction devices 30 and 40 that respectively suction the first and second suction devices 30 and 40 are removed and reflow treatment is performed thereon to adhere the solder balls 22 to the bonding pads 12, thereby completing flip chip bonding of the two chips.
FIG. 2 is a sectional view illustrating a structure of a cMUT 100.
As illustrated in FIG. 2, the cMUT 100 is manufactured by sequentially forming a lower electrode 120 and an insulating layer 130 on a semiconductor substrate 110 used in a general semiconductor manufacturing process, forming air gaps 140 on the insulating layer 130, and forming thin films 150 having a thickness of several to thousands of Å and an upper electrode (not shown because the thickness thereof is far smaller than that of the thin films 150) over the air gaps 140. In this regard, the semiconductor substrate 110 and the thin films 150 form a capacitor with the air gaps 140 formed therebetween. Each of the air gaps 140 is defined by a support member 160 made of a dielectric, and the thin films 150 supported by the support members 160 are formed over the respective air gaps 140. That is, the number of the thin films 150 corresponds to the number of the air gaps 140.
To electrically connect an integrated circuit such as an application-specific integrated circuit (ASIC) to cMUTs, a flip chip bonding technology, which is a core technology for packaging, may be applied. In the flip chip bonding process, there should be no problem with handling of chips, i.e., vacuum suction of chips.
To flip-chip bond a cMUT to an integrated circuit, bonding pads are formed on the side of a semiconductor substrate of the cMUT on which thin films are not formed, the surface of the cMUT on which the bonding pads are formed faces up, and the surface of the cMUT on which the thin films are formed faces down. The surface of the cMUT on which the thin films are formed is suctioned using a vacuum suction device to fix the cMUT, and the integrated circuit is aligned with the cMUT such that solder balls of the integrated circuit contact the bonding pads of the cMUT.
However, the thin films of the cMUT are very thin and thus are easily affected by external force. Thus, when vacuum pressure is applied to the thin films to suction the thin films in a flip chip bonding process, the thin films are easily damaged. That is, due to the characteristics of the thin films of the cMUT, it may not be possible to suction the surface of the cMUT on which the thin films are formed such that flip chip bonding may not be performed.
In addition, a generally used flip chip bonding technology is mainly bonding of chips in a one-to-one correspondence manner (bonding in a 1:1 manner). However, when at least two chips (chips B, C, . . . ) need to be bonded to a single chip A, the chip B is first bonded to the chip A, and then the remaining chips are consecutively bonded to the chip A one by one.
In this case, however, even though one of the cMUTs is first bonded to the integrated circuit in spite of damage to the thin films, it is difficult to bond the remaining cMUT(s) to the integrated circuit due to tilting of the bonded chip, resulting in reduced bonding accuracy. In addition, after bonding a single cMUT to the integrated circuit, it is difficult to perform flux dipping only on solder balls of the integrated circuit.